An estimated 4 to 5 million Americans (about 2% of all ages and 15% of those older than 65) have some form and degree of cognitive failure. Cognitive failure (dysfunction or loss of cognitive functions, the process by which knowledge is acquired, retained and used) commonly occurs in association with central nervous system (CNS) disorders or conditions, including age-associated memory impairment, delirium (sometimes called acute confusional state), dementia (sometimes classified as Alzheimer's or non-Alzheimer's type), Alzheimer's disease, Parkinson's disease, Huntington's disease (chorea), mental retardation (e.g. Rubenstein-Taybi Syndrome), cerebrovascular disease (e.g. stroke, ischemia), affective disorders (e.g. depression), psychotic disorders (e.g., schizophrenia, autism (Kanner's Syndrome)), neurotic disorders (i.e. anxiety, obsessive-compulsive disorder), attention deficit disorder (ADD), subdural hematoma, normal-pressure hydrocephalus, brain tumor, head or brain trauma.
Cognitive dysfunction is typically manifested by one or more cognitive deficits, which include memory impairment (inability to learn new information or to recall previously learned information), aphasia (language/speech disturbance), apraxia (impaired ability to carry out motor activities despite intact motor function), agnosia (failure to recognize or identify objects despite intact sensory function), disturbance in executive functioning (i.e. planning, organizing, sequencing, abstracting).
Cognitive dysfunction causes significant impairment of social and/or occupational functioning, which can interfere with the ability of an individual to perform activities of daily living and greatly impact the autonomy and quality of life of the individual. Thus, there is currently a need for compounds and methods that are useful for improving cognitive function in animals.
Phosphodiesterases (E.C. 3.1.4.17) are a class of enzymes that catalyze the hydrolysis of the 3′-phosphodiester bond of 3′,5′-cyclic nucleotides. The phosphodiesterase 4 (PDE4) isoform specifically hydrolyzes adenonsine 3′,5′ cyclic monophosphate (cAMP) to form 5′-adenosine monophosphate (5′-AMP). cAMP is a well studied intracellular second messenger that is known to be responsible for regulating a number of cellular processes including transcriptional regulation. One signaling pathway known to be regulated by intracellular levels of cAMP is the CREB pathway. The CREB pathway is responsible for regulating transcriptional activity in the brain (including the hippocampus) that leads to protein syntheses required for learning and memory, especially the consolidation of short-term to long-term memory. It is known that inhibition of PDE4 improves cognitive function in mammals, including contextual memory and object recognition (Tully, et. al., Nature Reviews Drug Discovery, 2003, 2, 267-277; and Barad, et al., Proc. Natl. Acad. Sci. 1998, 95, 15020-15025). It has also been shown to improve memory in animals with impaired CREB function (see Bourtchouladze, et. al., Proc Natl Acad Sci USA, 2003, 100, 10518-10522).
Numerous companies have invested in the development of specific PDE4 inhibitors to treat a variety of diseases, most notably in the anti-inflammatory field (e.g. Rolipram™, and Ariflo™). A common side-effect of these treatments has been the induction of emesis. Accordingly, there is a particular need for PDE4 inhibiting compounds that cause little or no emesis.